GEOLOGY AND PALEONTOLOGY OF THE UPPER JOHN DAY BEDS, JOHN DAY RIVER VALLEY, OREGON: LITHOSTRATIGRAPHIC AND BIOCHRONOLOGIC REVISION IN THE HAYSTACK VALLEY AND KIMBERLY AREAS (KIMBERLY AND MT. MISERY QUADRANGLES)

Abstract The John Day Formation of north-central Oregon preserves a succession of speciose, superposed Oligocene through early Miocene mammalian faunas that establish the sequence of mid-Cenozoic mammalian evolution within the Pacific Northwest. Upper John Day rock units initially described by Merriam (1900, 1901) in the Kimberly and Haystack Valley areas were later divided into lower (Kimberly) and upper (Haystack Valley) members by Fisher and Rensberger (1972). We focused our study on the lithostratigraphic succession within the Haystack Valley Member. Rocks previously included in the Haystack Valley Member can be subdivided into four unconformity-bounded, genetic lithostratigraphic units that range in age from ∼24 to ∼18 Ma, three of the units incorporating age-diagnostic mammalian faunas. We have identified two principal depositional units within the Haystack Valley Member of Fisher and Rensberger south of Kimberly: (1) Johnson Canyon Member—late or latest Arikareean tuffaceous siltstones and fine san...